The present invention relates to amplifier circuits and, more particularly, to a bipolar-CMOS differential amplifier stage that can common mode to ground and which may be utilized as a standard amplifier cell in designing and manufacturing integrated circuits.
Differential amplifiers that can common mode to substantially ground utilizing a single power supply are well known in the art. For example, U.S. Pat. No. 3,649,846 discloses such an amplifier which may also be used as a comparator for comparing input voltages having a common mode voltage range extending to zero volts. Many standard operational amplifiers, which are manufactured by Semiconductor manufacturers, use the teaching of the aforementioned patent. Although the foregoing referenced amplifier works quite well it does suffer from some drawbacks which are addressed by the present invention.
Amplifiers made in accordance with the teaching of the cited patent are constructed using bipolar transistors and bipolar techniques and require the use of the PNP transistor followers to the input of the differential amplifier stage. This limits the closeness that the input differential stage can operate to the positive supply rail as well as the circuit offset voltage. In addition, the bipolar process requires greater die area to be utilized to manufacture the amplifier in integrated circuit form than required by other processes, i.e., CMOS or BiCMOS. Further, it is desirable to lower device count.
Hence, a need exists for an improved amplifier stage that can be used as a standard cell for designing and manufacturing integrated circuits and which will common mode to ground without the need of input follower devices as well as operate near the positive supply rail.